THE CARBON MICROPHONE 



167 



phones (Fig. 8), the immediate ancestor of the granular carbon type 

 used today. Hunnings used powdered "engine coke." It carried 



Fig. 4 — Carbon-to-carbon single contact transmitter brought out in 1879 by Berliner. 



more current than the Blake transmitter but it was liable to "pack" 

 and become insensitive. 



This difficulty was overcome in the design invented by White in 

 1890, called the solid back type (Fig. 9). Millions of these are used 

 today in the ordinary desk-stand instrument. In this, carbon granules 



Fig. 5 — Nail contacts used by Professor Hughes in 1878 to demonstrate their micro- 

 phonic properties. 



